I don’t know why you might need this but its definitely better than using the Logon screen ( + L). In fact you can have a look what’s happening behind even if your desktop screen is locked.
ClearLock is a transparent screen locker aimed to make the computer tech/system admin’s live easier by providing a way to disable input to a computer while still allowing you to see what is going on. It is completely portable an can be run in a PE environment or on a live system without leaving anything behind.
# Download ClearLock: Link
ClearLock is not a security system. its purpose is to keep meddling users from screwing with the computer when you are fixing it. It will not keep your computer/data safe from someone who is determined to get at it.